“Black carbon” flowing from the soil to oceans

A smaller proportion of black carbon created during combustion will remain in the soil than previously estimated. Contrary to previous understanding, burying black carbon in the ground in order to restrain climate change will not create a permanent carbon reserve. Instead, a part of black carbon will dissolve from the soil to rivers. The flux of dissolved black carbon from the rivers to the ocean was estimated in a research article published in Science on 19 April. The research was funded among others by the Academy of Finland.

The burning of organic matter creates 40–250 million tons of black carbon every year. Black carbon is formed through the incomplete combustion of organic matter, for example, in forest fires, slash-and-burn and controlled burning of fields. The general assumption has been that black carbon would remain in the soil even for millions of years.

However, recently published research indicates that a remarkable proportion of black carbon in the soil will dissolve to the water system. In the light of new research results, the much discussed “biocarbon” may not be as beneficial in terms of mitigating climate change. Carbon is given the prefix “bio” when it is used both for energy production and soil enrichment. In any case, the stability of carbon in the soil has been a central factor of biocarbon applications.

By sampling rivers all around the world, the researchers estimated that the annual amount of black carbon flowing via rivers to the ocean is 27 million tons per year.

“Each sample included a significant amount of black carbon,” says a research participant Anssi Vähätalo, Senior Lecturer at the University of Jyväskylä.

“On average, the amount of black carbon was 10 per cent of the amount of dissolved organic carbon. The results prove that the proportion of water-soluble black carbon may be as much as 40 per cent of black carbon created annually.”

Water samples from the largest rivers in the world

The basis of the research was the ‘Big river’ project started by Senior Lecturer Anssi Vähätalo while he was working as an Academy Research Fellow at the University of Helsinki, before moving to the University of Jyväskylä. For this project, water samples were collected from the ten largest rivers in the world.

“These rivers carry a third of the freshwater running to the oceans, and their catchment area covers 28 per cent of the whole land area in the world. Water samples were taken, for instance, from the Amazon, the largest river in the world,” says Vähätalo.

In addition to the samples used in the river project, the research published in Science was supplemented with samples from many other rivers all over the world. The total number of researched samples was 174.